Except that by the time the stories were written down any alleged witnesses would most likely have been dead.
The average life expectancy for men at the time was 29, according to several internet sources I've looked at. Taking into account the number of infant and youth deaths, those who reached adulthood probably lived to 35 or 40. By the way, this explains why young girls would have been married off far younger than they are now, and why Mary is described as a young girl.
If these people, and the events they were alleged to have witnessed, were invented decades after the fact at a time when the majority of people had a life expectancy of perhaps forty-five years, who would have been around to question?
Do you have any evidence to show that 'these people, and the events they were alleged to have witnessed, were invented decades after the fact'. Remember that Paul had already been preaching for some time before he wrote his first letter (Galatians - believed by the majority of academics to have been written between 43 and 53 AD - 10 to 20 yerars after the events). Furthermore, the apostles had been preaching the message since a month after the death and resurrection events recorded in the documents.
Assuming, of course, that the written accounts came to light in the same area as the people lived, given that as well as living shorter lives people rarely travelled any distance.
In fact, this is a good argument for the validity of the 'stories' because those who were allegedly present at the events would have been able to be questioned by the authorites, by the followers of the apostles, etc. The locals would also have been able to inform the questioners if such and such a person had never existed.
Given a credulous general populace exposed to any number of tales of supernatural goings-on, why would they particularly question another tall tale in that environment?
How many 'tales of supernatural goings-on' would they have been exposed to? Do you have access to any such tales that would have been doing the rounds at the time?